The present invention relates to a throttle control apparatus for an internal combustion engine, and more particularly to a throttle control apparatus for an internal combustion engine in which the opening angle of a throttle valve is electrically controlled by a motor or the like thus preventing a sudden increase in engine torque caused by a throttle valve opening operation at the time of generation of an abnormality in the control system.
In recent years, many throttle control apparatuses have been proposed which electrically control the opening angle of a throttle valve of an internal combustion engine by a motor or the like. The characteristic of the throttle opening angle for the amount of accelerator operation can be set arbitrarily, and these control apparatuses have an advantage that it is possible to accurately cope with the running condition of a vehicle such as acceleration request. In contrast with a general system in which a throttle valve is mechanically coupled with an accelerator pedal, such throttle control apparatuses may possibly drive a throttle valve in an opening direction irrespective of the amount of accelerator operation when an abnormality is generated in a control system. In this case, there is an inconvenience in that an engine torque is suddenly increased against the driver's intent, thereby producing an uncomfortable feelng in the driver.
Throttle control apparatuses disclosed by JP-A-62-35039 and JP-A-60-159346 are known as throttle control apparatuses for an internal combustion engine capable of coping with the abnormality of a throttle control system.
In the throttle control apparatus disclosed by the JP-A-62-35039, when an abnormality is generated in a throttle control system in a state in which an engine rotation speed exceeds 2000 rpm, fuel is cut-off for each cylinder in an off-and-on way to limit the engine rotation speed to a value not higher than 2000 rpm, thereby preventing a sudden increase in engine output torque.
On the other hand, the throttle control apparatus disclosed by the JP-A-60-159346 copes with an abnormality in a throttle control system as follows. For example, when a throttle valve is opened notwithstanding that an accelerator is not operated, it is assumed that an abnormality is generated in a control system. As a result, the operation of three of four cylinders of an internal combustion engine is ceased (or put out of operation) by the fuel cut-off, thereby suppressing an engine output torque. When the accelerator is operated, lone cylinder being subjected to fuel cut-off is recovered, thereby ensuring sufficient engine torque to the extent necessary to continue the travel of the vehicle.
In the two kinds of throttle control apparatuses as mentioned above, a sudden increase in engine torque is suppressed by fuel cut-off when an abnormality is generated in a throttle control system. However, since the fuel cut-off is carried out by a fuel control system similar to the throttle control system, the possibility exists that the fuel would not be carried out due to the influence of the abnormality of the throttle control system. Therefore, the fuel cut-off is not sufficiently reliable as a countermeasure when an abnormality is generated.